Q: Here's a good puzzle: how do you write a program which produces its own source code as output?
A: It is actually quite difficult to write a self-reproducing program that is truly portable, due particularly to quoting and character set difficulties.
Here is a classic example (which ought to be presented on one line, although it will fix itself the first time it's run):
char*s="char*s=%c%s%c;main(){printf(s,34,s,34);}";
main(){printf(s,34,s,34);}
(This program
has a few deficiencies,
among other things
neglecting to
#include <stdio.h>,
and
assuming
that the double-quote character " has the value 34,
as it does in ASCII.)
Here is an improved version, posted by James Hu:
#define q(k)main(){return!puts(#k"\nq("#k")");}
q(#define q(k)main(){return!puts(#k"\nq("#k")");})
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